In that many radiotelephones are battery powered devices, an important consideration is in maximizing the operating time of the radiotelephone between required battery rechargings. This can be accomplished in part by periodically placing the radiotelephone, hereinafter referred to as a mobile station, into a sleep state. When in the sleep state a significant portion of the mobile station's circuitry is either powered off completely, or is operated at a reduced power level. Since the RF receiver, transmitter and related circuits consume a significant amount of power, it is desirable to especially power these components down or off. However, when the receiver circuitry is powered down or off the mobile station is temporarily disconnected from the cellular network. As such, it is important to insure that, when in the sleep mode, the mobile station does not miss any messages and/or control information that is required by the mobile station.
Reference can be had to commonly assigned U.S. Pat. No. 5,471,655, issued Nov. 28, 1995, entitled "Method and Apparatus for Operating a Radiotelephone in an Extended Stand-by Mode of Operation for Conserving Battery Power", by Raimo Kivari, for teaching various embodiments of a mobile station that is operated in a low power, discontinuous reception mode of operation.
Referring to FIG. 4, it has been proposed for EIA/TIA IS-91-A (i.e., analog (FM) mobile stations) to provide the illustrated forward control channel frame configuration to support the sleep mode. The basic idea of this proposal is to divide the control messages, which are transmitted to the mobile stations operating in idle sleep mode, into eight paging slots. A mobile station operating in idle sleep mode wakes at the beginning of its assigned paging slot to receive any control messages addressed to it, the paging slot being determined by the mobile station's telephone number. The mobile station goes back to sleep when it receives a control-filler message. As such, the wake up time (the period for receiving mobile station control messages) can be longer than the paging slot shown in FIG. 4. Additionally, the mobile station must wake periodically to listen to overhead messages, which are transmitted for all mobile stations listening to the same base station.
In this proposal the forward control channels (i.e., base station to mobile station) are configured for sleep mode by being organized in the illustrated frame format. The illustrated configuration must be used if a Registration Increment Global Action Message is sent on this control channel with an Idle Speep Mode (ISM) field set to `1` (see IS-91-A, Section 3.7.1.2.2). The ISM field in the Registration Increment Global Action Message indicates whether the mobile station can operate on the idle sleep mode on the control channel currently being monitored. Each word in the frame consists of the dotting pattern, word synchronization, busy/idle bits, and five repeats each of the stream A and stream B message words.
The proposed organization of the forward control channel for support of sleep mode is as follows. Each frame in stream A and B consists of 23 word positions. Any otherwise unfilled word positions are occupied with a control-filler message (see IS-91-A, Section 3.7.2.4). The first two words (word positions 0 and 1) of each channel frame are always the two words of the system parameter overhead message (see IS-91-A, Section 3.7.1.2.1). Up to five global action overhead messages (see IS-91-A, Section 3.7.1.2.2) and/or the registration ID message (see IS-91-A, Section 3.7.1.2.3) and/or other messages are sent sequentially in the next words within the frame, beginning with word position 2.
The mobile station listens to either stream A or stream B messages depending on the last bit of its telephone number. The AMPS/NAMPS related explanations in this patent application exclude, for simplicity, the messages of the stream not meant for the mobile station.
Mobile station control messages (see IS-91-A, Section 3.7.1.1) that are directed to mobile stations not operating in an idle task, and/or those mobile stations that were sent, within the preceding four seconds, an Awake Hold order (see IS-91-A, Table 3.7.1.1-1) may be placed in any of the word positions 7-23, or in any of the word positions 2-6 not occupied by messages that are part of the overhead message train (OMT). Mobile control messages are specified to not extend into the next forward control channel frame.
However, a problem is created by this proposed technique in that different mobile stations are operated so as to receive different ones of the OMTs and, as such, unless the same information is transmitted in several OMTs, some mobile stations may miss important information.